The present invention relates to a tolerance conversion technology using computer-aided design (CAD) data.
Japanese Patent Laid-open Publication (kokai) 5-225290 teaches a common tolerance conversion technology for converting shapes using a parametric function.
Achieving an integrated CAD/CAM (computer-aided manufacturing) system capable of generating numeric control (NC) data for machine tool control based on CAD data generated from the design requires a tolerance conversion step for converting shapes by converting the nominal dimensions specified by the design to process dimensions determined in consideration of a defined tolerance. However, if there is a problem in a manner that the process dimensions are determined in the process dimensions, the tolerance conversion step can create a shape which is contrary to the user's intention and requires one or more extra processing steps, thus increasing the production cost. Such shapes are referred to herein as “inappropriate shapes”.
This is further described below with reference to FIG. 2 and FIG. 3, which show examples of such inappropriate shapes.
In the example shown in FIG. 2 the design angle between surfaces 201 and 202 is perpendicular, but is changed to an acute angle as a result of tolerance conversion. In addition to the increased production cost of parts in which the angle between two such surfaces changes from perpendicular to acute, the finished shape can also be dangerous, thus requiring an additional process.
In the example shown in FIG. 3 surfaces 301 and 302 should be on the same plane but are offset at different heights. The part before shape conversion enables surfaces 301 and 302 to be processed in a single operation using a single large tool. Channel 303 can then be formed using a smaller tool. The part before shape conversion thus provides greater freedom in tool selection, making It possible to suppress the production cost. However, production cost is increased by the part resulting from shape conversion, however, because the change in surface positions reduces the degree of freedom in both tool selection and processing methods.
Changes such as these to an inappropriate process shape resulting from tolerance conversion as described above are difficult to find by visual inspection because the changes are so small.
Furthermore, if detection precision is defined as (number of changes between shape elements inappropriate to processing)/(number of changes between all detected shape elements), changes in shape elements inappropriate to processing will also be detected if shape elements are simply compared, and the number of detections will increase. This is because the changes in inappropriate shape elements produced by tolerance conversion are numerous between particular shape elements, and this is not considered.